We propose a longitudinal population-based study to identify modifiable factors associated with becoming overweight or obese. We will provide new evidence on the factors associated with the maintenance of stable weight and body mass index (BMI) in adults. Further, we will examine the dynamic relationships among physical activity, weight, health-related quality of life (HRQL), and the physical and social environment. Data come from a major longitudinal population health survey, the Canadian National Population Health Survey (NPHS), for the period 1994/95-2006/07. Analyses will exploit data on a cohort of 13,045 adults (18+) interviewed in 1996/97 and every two years thereafter, with follow-up data through 2006-07. Analyses will be conducted using a determinants of population-health framework in which demographic factors, number of chronic health conditions, health behaviors, psychological health, psychological resources, social resource and other variables known to be associated with health are included. Further, in this framework, we will investigate the importance of the physical and social environment in explaining the trends in weight and BMI. Analyses will be conducted separately for men and women. The research will be conducted by a multi- disciplinary team of investigators. There are two primary aims for the exploratory two-year project. Aim #1. In a longitudinal cohort of 13,045 adults, we will investigate the factors that predict the maintenance of stable weight, weight gain, and weight loss over a 10-year follow-up period. We will also investigate the factors that predict change (or lack thereof) in BMI category (underweight, normal, overweight, and obese). Aim #2. We will investigate the relationships among physical activity, BMI, the built environment, the social environment, and HRQL. In particular, we will examine the temporal relationships between changes in physical activity and changes in BMI and HRQL. PUBLIC HEALTH RELEVANCE: Obesity is a health risk. Little is known about the patterns of change in physical activity, weight, and health- related quality of life over time. Findings from this study will identify modifiable risk factors for obesity and overweight.